RE: [Algorithms] Generation texture co-ordinates for decals

[Chris B. (BUNGIE) <cbu...@mi...>]

The truly insane might also like to consider a spring-relaxation
technique that runs in real-time after the projectile hits the surface.
Goop gun, anyone?

--
Chris Butcher
Rendering & Simulation Lead
Halo 2 | Bungie Studios
bu...@bu...
=20

-----Original Message-----
From: Charles Bloom [mailto:cb...@cb...]=20
Sent: Tuesday, August 27, 2002 13:44
To: gda...@li...
Subject: RE: [Algorithms] Generation texture co-ordinates for decals


If you were insane, you would gather the geometry that you want to
put the decal on, generate a local non-distorting 2d mapping around
the "root point" to decal, and make uv's that way.  Basically you're
generating a surface->2d local mapping such that the geometric area
covered is proportional to the texture area.  Anyway, sounds like a
pre-process thing, not a real-time thing.

At 01:02 PM 8/27/2002 -0700, Chris Butcher (BUNGIE) wrote:
>   For Oni we ended up literally wrapping the decal's texture
coordinate
>system across an edge if the angle was large enough. This meant that
the
>texture would exhibit seams on some edges (can't fold a sticker to
>exactly cover a sphere, for example) so it wasn't appropriate for
>anything that wasn't basically scorch-like. A priority queue helps so
>that you wrap over small inclines preferentially, leaving the seams for
>the more right-angle type surfaces.
>
>   This was probably more difficult than it needed to be. For Halo we
>simply used the projective texturing approach, with a simple heuristic
>to determine whether we should try to wrap across a polygon (skipping
>anything that was too oblique to the decal's incoming vector). Remember
>that to generate the decal in the first place, you probably don't
>exactly want the surface normal, and you definitely don't want the raw
>incoming projectile vector. Something between the two worked well for
>us.
>
>--
>Chris Butcher
>Rendering & Simulation Lead
>Halo 2 | Bungie Studios
>bu...@bu...
>
>
>-----Original Message-----
>From: Ratcliff, John [mailto:jra...@so...]
>Sent: Tuesday, August 27, 2002 11:33
>To: gda...@li...
>Subject: [Algorithms] Generation texture co-ordinates for decals
>
>I thought I had this under control but I am still experiencing some
>issues.
>When I searched the web for 'decals' most of the examples that showed
up
>used a projection matrix to generation u/v co-ordinates for a decal.
>
>This is how I wrote my code, and I then clipped the geometry against
the
>texture space co-ordinates.  This worked    ok, in general, except for
>polygons which were at oblique angles to the view vector.  There the
>decal
>texture would stretch in a very ugly fashion.
>
>I decided to look for reference material that showed how to generate
U/V
>co-ordinates for decals.  In "Game Programming Gems 2" there is a brief
>blurb in section 4.8 about one way to do this.  The section comes with
a
>tiny source code fragment as well.
>
>However, when I used the equation provided there to generate u/v
>co-ordinates I had similar problems.  At oblique angles the texture
>co-ordinates would approach infinity.
>
>When you read the book chapter it has a section about clipping the
>geometry,
>but it doesn't say whether you *must* have the geometry clipped
*before*
>you
>generate the texture co-ordinates.  With my old system I clipped the
>geometry to texture space, not in object space.  I don't mind clipping
>the
>geometry in object space, and then generating the texture co-ordinates,
>if
>that's what I have to do to get this to work.  However, I don't
>understand
>the math well enough to know for sure this will make any difference at
>all.
>
>Does anyone have any familiarity or stategies for how to wrap a decal
>around
>some arbritrary geometry without it shooting off to infinity at oblique
>angles?
>
>John
>
>
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Charles Bloom    cb...@cb...    www.cbloom.com



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